The present invention has particular application to breath alcohol testing devices such as the ones sold by Intoximeters Inc., 1901 Locust Street, St. Louis, Mo., under the trademark ALCO-SENSOR, and especially to such devices equipped with fuel cells constructed as described in Wolf U.S. Pat. No. 4,487,055, but its use is not limited thereto.
In breath alcohol testing devices presently used commercially, in which fuel cells are employed, the conventional way of determining breath alcohol is to measure a peak voltage across a resistor due to the flow of electrons obtained from the oxidation of breath alcohol on the surface of the fuel cell. There are a number of problems. The peaks become lower with repeated use of the fuel cell. The peaks vary with different temperatures. In order to produce a high peak, it is customary to put across the output terminals of the fuel cell a high external resistance, on the order of a thousand ohms, but the use of such a high resistance produces a voltage curve which goes to the peak and remains on a high plateau for an unacceptably long time. To overcome that problem, present systems provide for shorting the terminals, which drops the voltage to zero while the short is across the terminals. However, it is still necessary to let the cell recover, because if the short is removed in less than one-half to two minutes after the initial peak time, for example, the voltage creeps up. Peak values for the same concentration of alcohol decline with repeated use whether the terminals are shorted or not, and require 15-25 hours to recover to their original values.
The individual fuel cells differ in their characteristics. All of them slump with repeated use in quick succession and also after a few hours' time of non-use. They degrade over time, and in the systems used heretofore, must be re-calibrated frequently. Eventually, they degrade to the place at which they must be replaced. Presently, the cell is replaced when it peaks too slowly or when the output at the peak declines beyond practical re-calibration, or when the background voltage begins creeping excessively after the short is removed from the cell terminals.
One of the objects of this invention is to provide apparatus and method that provide a measure of breath alcohol that is largely independent of the value of the peak voltage.
Another object is to provide such a device and method that require no recovery time beyond the period of measurement.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.